1. Field of the Invention
The present invention relates to a heat exchanger for a refrigerant cycle in which carbon dioxide (CO.sub.2) circulates. More particularly, the present invention relates to a method for manufacturing a header tank of the heat exchanger.
2. Description of Related Art
In the recent years, instead of flon, CO.sub.2 is used as a refrigerant in a refrigerant cycle (hereinafter, referred to as CO.sub.2 refrigerant cycle). In the CO.sub.2 refrigerant cycle, a pressure of CO.sub.2 refrigerant at a discharging side of a compressor exceeds the critical pressure, and is approximately 10 times of a pressure of flon refrigerant at the discharging side of the compressor. Therefore, in the CO.sub.2 refrigerant cycle, a sufficient strength is necessary at a brazing portion of a heat exchanger. For example, in a conventional radiator described in JP-U-55-10073, as shown in FIGS. 7A, 7B, insertion holes 25 into which flat tubes 11 are inserted are formed in a tank plate 20a of a header tank by press-punching, and a header cover 20b is brazed to the tank plate 20a. Therefore, it is difficult for the conventional radiator to be used in the CO.sub.2 refrigerant cycle.